Optical systems are commonly used in ground-based surveillance systems and in guidance systems of aircraft and missiles to identify and track other objects or aircraft. Such surveillance and guidance or “seeker” systems may be based on infrared detectors (IR), i.e., those that detect light in the infrared region of the spectrum. Such surveillance systems are often referred to as forward looking infrared systems or “FLIRs.” Forward looking infrared imaging systems may be divided into two broad categories, namely, scanning systems and non-scanning or staring systems. For missile guidance there are also non-imaging systems based on a single detector element and modulating the light from a target image.
Staring systems have an image field of view that is fixed in relation to a focal plane of the system. In contrast, scanning imaging systems have an optical element, for example a prism or mirror, that rotates or oscillates, causing an image captured by the system to move or “scan” past a focal plane of the imaging system. Typically systems from each category have infrared detector elements or pixels positioned at a focal plane of the system. Because an image of the field of view of the system is focused at the focal plane, a detector placed at the focal plane may detect optical intensities and consequently the presence of any object in the field of view. An array of detectors may be used in place of a single detector, in which case the array may be referred to as a detector array. When an array is positioned at a focal plane of the system, the image received by the array may be processed by digital imaging techniques and systems.
Examples of applications in which forward looking infrared systems are used include missile seeker systems, airborne threat warning systems, and infrared search and track systems. Forward looking infrared systems are also used for navigation, target acquisition, fire control, and reconnaissance on aircraft, ships, ground vehicles and man-portable systems. As an example of the prominence of these infrared systems, it has been reported that shoulder-fired, surface-to-air missiles guided by infrared seeker systems have accounted for 80–90% of the aircraft combat losses since the Persian Gulf War of 1991, including the only Army attack helicopter downed by Iraqi forces during that war.
As used in imaging systems, detector arrays may be connected to or integrated with suitable digital signal processing circuitry for image processing, e.g., display, reconstruction, filtering, manipulation, etc. The detectors used in such arrays are often charge-coupled devices (CCDs) or photodiodes. A common type of array used in such scanning imaging systems is a linear, one-dimensional or “1×N” array, referring to the geometry of the array being one row or column with N individual detectors. The terms row and column are relative and depend on the orientation of one viewing the array. For the sake of clarity, the term “column” or “line” will be used hereinafter. Arrays of more than one column may be used in scanning systems. These arrays may be referred to as two-dimensional or “2D” arrays and may be referenced by the number of columns they contain and the number of elements residing in each columns, e.g., 2×128, 4×64, etc. When two or more columns are present in an array, the signal from each element or pixel may be added and averaged over time in a process called time delay integration (TDI) to improve the signal to noise ratio of the detected image. Common FLIR detector materials include indium antimonide, InSb, and mercury cadmium telluride, HgCdTe.
Some forward looking infrared systems include detectors that have in-focal-plane electronics, which enable signal processing on each detector or pixel in the array. Signal processing capability on each pixel may provide imaging advantages, such as the opportunity to perform digital matched-filter processes, but also presents opportunities to perform countermeasures that disable the pixel or array. It has been demonstrated that infrared detector arrays may be damaged and disabled by directing a laser beam at a detector element or pixel in the array.
When focused by imaging optics onto a detector array, a laser beam or pulse, if having sufficient fluence, can cause laser sparks, which in turn can produce laser-induced damage of the array material. Such resulting laser sparks may remove material by melt, ablation, vaporization and may “drill” down through the infrared detector material to the underlying processing electronics. This phenomenon may be referred to as laser-material interaction. Disablement of the pixel or array may occur when a spark induces thermal damage and/or mechanical damage or causes a quantity of charge to move though the layers of the detector into an underlying layer, e.g., readout integrated circuitry (ROIC), multiplexing circuitry, integration circuitry, etc. Any of such phenomenologies resulting from a laser spark can causes either an open circuit or a short circuit in one of the layers under the detector or array, causing a total shutdown of the functionality of the array. The focused beam or pulses may further interfere with the normal operation of the array by producing heated plasmas at the detector surface that may induce disruptive voltages and currents.
As previously stated, forward looking infrared systems using both scanning or non-scanning systems may employ detector arrays or focal plane arrays, where a focal plane array usually implies multiple columns of detectors. Staring systems employing focal plane arrays may be disabled by a continuous-wave (CW) or pulsed laser beam with sufficient fluence when the beam is focused in the focal plane. The focal plane array is always positioned at the focal plane of such a staring system, and the focal plane is fixed in relation to the focal plane array. Therefore, as long as the laser beam or pulse travels through the imaging system, the beam or pulse will be focused on the focal plane and any array positioned there. This is not the case with scanning imaging systems, however, since the image of the field of view is not fixed and is instead scanned past the focal plane, where the detector array is located. Therefore, there is no guarantee that at any particular time an incident laser beam will hit an array in the focal plane of the imaging system. At any particular time, the position on the focal plane where an incident laser beam is focused depends on the orientation of the moving optical element, e.g., prism or mirror. As a result, an incident laser beam or pulse may be focused on an area of the focal plane that at the moment critical for disablement does not include the detector array, despite the beam or pulse having entered the optical imaging system.
Besides being disabled by material damage from a direct hit through laser-detector interaction, an array may also be at least temporarily disabled by the application of a laser beam not directly on but near to the array, for example on whatever material is adjacent to the array. This temporary disablement condition may be referred to as a “latch-up” condition. The latch-up condition is not a damage condition but the functionality of the array is lost until the array system is rebooted. For a more detailed description of latch up, see “Understanding Latch-Up in Advanced CMOS Logic,” Application Note, Fairchild Semiconductor Corporation (Revised 1999).
Countermeasures have been developed to attempt to defeat the seeker systems of “guided” missiles. U.S. Pat. No. 5,703,314 issued to Meeker discloses a countermeasure system, adapted for use onboard an aircraft, for confusing an incoming missile as to the location and heading of the aircraft. The countermeasure system generates for each side of the aircraft at least two infrared energy images, which are projected onto the aircraft's fuselage and then swept across the aircraft's fuselage to confuse the incoming missile's infrared seeker. This system does not disable a detector in the guidance system of the missile.
U.S. Pat. No. 6,369,885 issued to Brown et al. discloses a missile tracking and deflection system for protecting a platform that includes a missile warning system for detecting the presence of a missile and generating a warning signal. A countermeasure processor receives the warning signal and analyzes characteristics of the missile to prioritize a trajectory signal. The countermeasure processor generates a jam code delivered by a laser beam to divert the trajectory of the missile away from the platform. U.S. Pat. Appl. Pub. No. U.S. 2002/0097390 with inventors Hick et al. is a continuation-in-part of U.S. Pat. No. 6,369,885 and further discloses that a nulling or blanking signal may be used during generation of the laser beam to improve reception of the active signature. Neither of the disclosed systems disables a detector in the guidance system of the missile.
While CW lasers producing a beam of sufficient fluence might hit and disable a focal plane array within a scanning imaging system, the average power requirements to ensure disablement could require a laser and power source that are prohibitively large for aerospace applications where mass is a critical consideration. Therefore, there is a need for a system and processes for disabling scanning imaging systems by use of a pulsed laser having a carefully controlled pulse repetition frequency.